Highly thermoresistant polymers are required in microelectronics, particularly as protective and insulating coatings and as dielectrics (see, for example, "SAMPE Journal" 25, No. 6, pp. 18-23 (1989) and "Proceedings of the 1992 International Conference on Multichip Modules", pp. 394-400). Some of the polymers used, such as homo- and copolymers of aromatic polyethers and precursors of polyimides (PI) and polybenzoxazoles (PBO) show a good solubility in organic solvents and good film-forming properties and can be applied to electronic components by means of spin-coating technology (see, for example, "High Performance Polymers" 4, No. 2, pp. 73-80 (1992) and "Polymers for Advanced Technologies" 4, pp. 217-233 (1993).
By means of a temperature treatment, polymer precursors of the above-mentioned type are cyclized, i.e., converted to the corresponding polymers (PI or PBO); this results in the final properties. This is because, as a result of the cyclization, the hydrophilic groups of the poly-o-hydroxyamide disappear, i.e., the NH, OH, and CO groups, which would have a negative effect on the dielectric properties and the water absorption. This is, for example, a significant advantage of the polybenzoxazoles as compared with the polyimides (with two CO groups per imide unit) and, in particular, as compared with the hydroxypolyimides (with two CO groups and one OH group per imide unit). In addition, the cyclization is important not only for the good dielectric properties and the low water absorption of the end product but also for its high temperature stability.
PI and PBO precursors can, for example, be adjusted photosensitively by the addition of suitable photoactive components and can then be structured directly, i.e., without the use of an auxiliary resist. This is important for the reason that direct structuring--as compared with indirect structuring--offers considerable cost advantages.
In contrast to most photosensitive PI precursors, photosensitive PBO precursors offer the advantages of a positive structurability, such as a lower defect density in the structuring of the so-called "via holes", because--in comparison with negative operating systems--only a fraction of the surface is exposed to light. The use of alkali-soluble PBO precursors also results in the possibility of an aqueous alkaline development. After the photostructuring, the cyclization of the precursors is then carried out by annealing.
PBO precursors that can be developed in aqueous alkaline medium are already known (see European Patent 0 023 662, European Application 0 264 678, and European Patent 0 291 779). The photolithographic process used, except for the cyclization, is the same as in the structuring of known positive resists based on novolaks and quinone azides, a process that is used in numerous production lines worldwide (see, for example, D. S. Soane and Z. Martynenko: "Polymers in Micro-electronics--Fundamentals and Applications", Elsevier, Amsterdam (1989), pp. 77-124).
The solubility of the PBO precursors in alkalies is an important requirement for their use as base polymers for photosensitive dielectrics that can be developed in aqueous alkalies. For microelectronic uses, the precursors must be soluble in developers free of metal ions, so that developers of this type can also be used in the photostructuring. This is because developers containing metal ions can have a negative effect on the electrical operation of the components.
The most common method for the preparation of alkali-soluble PBO precursors, i.e., of poly-o-hydroxyamides, is the reaction of a dicarboxylic acid chloride with a suitable bis-o-aminophenol. To capture the hydrogen chloride formed in the reaction, a soluble base, such as pyridine, is added, as a rule (see European Application 0 264 678 and European Patent 0 291 779). Although it is possible, by means of this method, to prepare precursors that are soluble in aqueous alkaline developers free of metal ions, there is the disadvantage that chloride ions remain in the polymer. However, a polymer of this type can not be used as a coating material for microelectronic components, because the chloride ions cause corrosion and can thus strongly impair the operation of the components. A purification of the polymer by means of ion exchangers is therefore required. However, this purification is time-consuming and expensive, as it includes additional process stages, such as the preparation of the ion-exchange column, dissolution of the polymer, passage of the solution through the column and rewashing, and repetition of the precipitation and drying.
In the preparation of poly-o-hydroxyamides, it is also necessary to meet the requirement that the dicarboxylic acid chloride react predominantly with the amino groups of the bis-o-aminophenol (with amide formation), but not with its hydroxy groups (with ester formation), i.e., the reaction selectivity of the amide formation must be high as compared with that of the ester formation. If the ester formation can not be excluded or strongly suppressed, then this will lead to insufficiently alkali-soluble polymers. A low reaction selectivity can also lead to a gel formation in the polymer solution, as a result of which the poly-o-hydroxyamide produced then becomes unfilterable and thus useless.
Processes for the chloride-free synthesis of poly-o-hydroxyamides--and also of poly-o-mercaptoamides--have also already been described. Thus, it is known, from European Application 0 158 726, to react dihydroxy- and dimercaptodiamino compounds with a dicarboxylic acid in the presence of a carbodiimide. However, in this reaction, urea residues remaining on the resin as a result of a rearrangement reaction frequently present problems. This is because they reduce the thermal stability of the polybenzoxazole or polybenzothiazole and the quality of the coatings prepared from these. In addition, the polymers produced by this process are not sufficiently soluble in aqueous alkaline developers free of metal ions.
An alternative chloride-free production process for poly-o-hydroxyamides consists of using condensation reagents, such as 1-ethoxycarbonyl-2-ethoxy- 1,2-dihydroquinoline and 1,1'-carbonyldioxydi-1,2,3-benzotriazole (see European Application 0 391 196) for the reaction of the dicarboxylic acid with the bis-o-aminophenol. However, the polymers produced in this manner also show an insufficient solubility in aqueous alkaline developers free of metal ions.
Processes are also known in which the amide formation takes place by means of phosphorus compounds (see: European Application 0 481 402, U.S. Pat. No. 4,331,592, and German Application OLS 3,716,629). However, in the case of poly-o-hydroxyamides, syntheses of this type lead either to cyclized, i.e. alkali-insoluble, products, or phosphorus-containing, in part also chemically bonded, residues remain in the polymer, as a result of which the polymer, because of the doping properties of phosphorus, becomes unusable for microelectronic applications. This is because, in contrast to ionic impurities, residues of this type can not be removed, e.g., by means of ion exchangers.